Short answers, pulled from the story.
An irregular galaxy lacks a distinct regular shape, nuclear bulge, or spiral arm structure that defines other cosmic categories. Stephen D. Butz noted in 2002 that such chaotic appearances place them outside the standard Hubble sequence while W. W. Morgan and N. U. Mayall described this absence of order in their 1957 spectral classification work.
Astronomers identified three major types including Irr-I galaxies which feature some structure but not enough for the Hubble sequence and Irr-II galaxies showing no structure capable of fitting into any established category. A dwarf irregular galaxy known as dI-galaxy represents a third distinct class highlighted by Eva K. Grebel in her 2004 research paper.
Uneven gravitational pull from neighbors can twist a stable shape into something unrecognizable while galaxy collisions also serve as a catalyst for creating these chaotic forms. These objects are commonly small measuring about one tenth the mass of the Milky Way though large exceptions exist like UGC 6697 which defies typical size constraints.
The absence of density waves allows researchers to study star formation in isolation without complex pressure patterns found in organized spirals. Walter F. et al. documented this phenomenon in their 2007 Astrophysical Journal article regarding local group dynamics where gas and dust behave differently than in standard systems.
NGC 1427A stands as an Irr-I category galaxy located about 52 million light-years away while the Large Magellanic Cloud was once classified as irregular before reclassification as type SBm. Corso and Buscombe published findings on its barred spiral structure in 1970 whereas the Small Magellanic Cloud remains an Im type despite containing a bar structure.